1. Field of the Invention
The present invention relates generally to the field of systems for measuring mercury in flue gas streams. More specifically, the present invention discloses a particulate separation system for mercury analysis in flue gas streams.
2. Statement of the Problem
Several approaches are currently employed to measure vapor-phase mercury in flue gas streams containing particulate matter. However, these techniques introduce sampling artifacts that are not adequately addressed by the current methods. Many particulates present in flue gas streams have an affinity for mercury and may remove vapor-phase mercury from the gas stream or convert mercury from one vapor-phase species to another. For example, fly ash particles have been shown to both remove mercury and oxidize mercury. Many current measurement techniques use a filter to separate the particulate matter from the gas stream. However, if the particulate matter separated from the gas stream is not inert, the vapor-phase mercury measured downstream from the filter will not accurately represent the mercury upstream from the filter.
An example of an inertial filter in the conditioning assembly for continuous stack monitoring was disclosed by the Bendix Corporation in U.S. Pat. No. 4,161,883 (Laird et al.). Mott Metallurgical Corporation also offers an inertial gas sampling filter similar to the Bendix inertial filter. The Mott filter effectively separates the majority of the particulate matter from the gas stream and filters the remaining particulate matter. If the gas stream contains particulates with a low affinity for mercury and a large fraction of particles that can be efficiently separated by means of an inertial filter (i.e., particles having a relatively large aerodynamic diameter), the Mott system should work well to provide a particulate-free gas stream for measurement of vapor-phase mercury without altering the species of mercury (i.e., particulate, oxidized vapor, or elemental vapor). However, for gas streams containing particles with an affinity for mercury or gas streams with a large fraction of small particles, the Mott system can introduce significant sampling artifacts from the thin layer of particulate matter collected on the filter surface.
3. Solution to the Problem
The present invention is a modified inertial filter that addresses the shortcomings associated with the prior art by including a heater and temperature controller that maintain the inertial filter within a predetermined temperature range. This enables accurate measurement of vapor-phase mercury without biasing the measurement due to desorption of particulate-phase mercury or absorption of mercury vapor onto fine particles that have an affinity for mercury.
This invention provides a particulate separation system for accurate measurement of vapor-phase mercury in a flue gas stream. The process uses an inertial gas sampling filter in which the skin temperature of the filter element is controlled to allow vapor-phase mercury measurements while minimizing measurement artifacts caused by: (1) mercury thermally desorbing off particulates into the gas stream; and (2) mercury being removed from the vapor phase by collection on particulate matter at the gas/particle separation interface.
These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings.